Transformer Sizing Calculator
Enter your load's current, voltage, and phase — get the calculated kVA and the recommended standard transformer size, with a safety margin for growth.
| Calculated load | |
|---|---|
| Target with margin | |
| Recommended standard size |
Get free quotes from vetted local electricians for panel upgrades, service work, and installs.
Get free quotes →We may earn a commission if you hire through this link, at no cost to you.
Standard transformer kVA sizes
1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 112.5, 150, 167, 200, 225, 250, 300, 333, 500, 750, 1000, 1500, 2000, 2500, 3000 kVA. Actual availability varies by manufacturer and transformer type (dry-type vs. pad-mount vs. pole-mount) — confirm your exact target size is in stock before specifying it. This is the fixed set of steps the calculator below always rounds up to; it's why the output is never an oddly specific number like "104 kVA."
How to use this calculator
Enter the load current the transformer needs to supply, the voltage, and whether it's single-phase or three-phase. Set a safety margin — 25% is a common default — and the calculator converts your load to kVA, applies the margin, and rounds up to the nearest standard manufactured transformer size.
Why size above the calculated load
A transformer running at 100% of its rated capacity continuously has no headroom for load growth and runs hotter over its service life, which shortens its lifespan. Sizing with a margin — commonly 25%, sometimes more for facilities expecting significant future load — is standard practice, not overengineering.
Worked example
A 100A, 480V three-phase load: calculated kVA is 1.732 × 480 × 100 / 1000 = 83.1 kVA. With a 25% margin, the target is 103.9 kVA — which rounds up to the next standard size, 112.5 kVA, since 100 kVA doesn't clear the margin target.
Frequently asked questions
How do I convert amps to kVA for transformer sizing?
Single-phase: kVA = (Voltage × Amps) / 1000. Three-phase: kVA = (1.732 × Voltage × Amps) / 1000. This calculator applies the correct formula automatically based on the phase you select.
What safety margin should I use for transformer sizing?
25% is a widely used default that balances headroom against cost — oversizing a transformer significantly raises upfront cost with little added benefit. Facilities with concrete plans for major load growth sometimes size higher; a simple, stable load might reasonably use less. If you're not sure, 25% is a safe default to start from.
Why isn't my calculated kVA a standard transformer size?
Transformers are manufactured in a fixed set of standard kVA ratings, not built to an arbitrary exact number — this calculator's job is converting your specific load into the right standard size to specify, rounding up rather than down so the transformer has adequate capacity.
Standard kVA sizes reflect common industry offerings; actual availability varies by manufacturer. Reference only — verify against manufacturer specifications and consult a licensed electrician or engineer before specifying equipment.